In Long Term Evolution (LTE) systems, advanced modulation has been introduced for use in downlink transmissions to increase the wireless device's data throughput when channel link quality is good. With the addition of advanced modulation, a downlink transmission from network node such as a base station or an evolved Node B (eNB) to a connected wireless device, such as a user equipment (UE), can be configured as either a higher order modulation (HOM) mode, where the modulation set used is, for example, {Quadrature Phase Shifting Key (QPSK), 16 Quadrature Amplitude Modulation (QAM), 64 QAM, 256 QAM}, or a legacy mode, where the modulation set used can be, for example, {QPSK, 16 QAM, 64 QAM}. Depending upon the downlink channel feedback from the wireless device, the base station, e.g., eNB, can select one of these download transmission modes.
Downlink transmissions to a wireless device can use 256 QAM modulation if the wireless device supports HOM mode and if the wireless device is configured in HOM mode. 256 QAM modulation configures each modulation symbol carry 8 bits of information, so that the peak throughputs can be up to 33% higher than using 64QAM where each symbol uses 6 bits.
The Third Generation Partnership Project (“3GPP”) defines different sets of mapping to be applied for wireless devices operating in HOM mode (referred to herein as “HOM capable wireless devices in HOM mode”) compared to wireless devices operating with non-HOM legacy modulations only. The wireless devices that can only operate with legacy modulation schemes can be wireless devices that are non-HOM capable wireless devices (referred to herein as “legacy wireless devices”) or wireless devices that are HOM capable wireless devices in legacy mode (referred to herein as “HOM capable wireless devices in legacy mode” or “HOM mode capable wireless devices in legacy modulation mode”).
Tables may be used for mapping Channel Quality Indicator (CQI) reports to signal to interference plus noise ratio (SINR) values. A HOM wireless device uses an alt-CQI table for reporting its channel quality, in which a CQI value maps higher modulations and code-rates than one from the legacy CQI table. Tables may also be used for mapping modulation and coding scheme (MCS) to transport block sizes. The mapping table for HOM wireless devices has fewer row entries for lower modulations and additional ones for 256 QAM.
A HOM-capable wireless device can be configured to operate in HOM mode or legacy modulation mode. Decisions are made by the base station, and in some embodiments, a Radio Resource Control (RRC) reconfiguration message is sent to the wireless device informing the wireless device of a modulation mode switch. According to 3GPP standards, Downlink Control Information (DCI) format 1A, the legacy modulation mode is applied for downlink data transmission during the mode transition period.
HOM-capable wireless devices may travel to a location, such as a cell edge, where the performance in HOM mode is worse than in legacy modulation mode due to lack of sufficient MCS options for QPSK ranges. Strategies are required for the base station to decide the best time for reconfiguring a HOM-capable wireless device with a preferable modulation mode.
While high order modulation provides higher throughputs, it requires cleaner radio channel conditions with low Error Vector Magnitude (EVM) tolerance, approximately <2%. Under a high EVM, data transmission with 256 QAM may result in a higher Block Error Rate (BLER), which in turn makes 256 QAM performance even worse than using a lower level modulation, such as 64 QAM.
The 3GPP standards specify an EVM of 3.5% for 256 QAM while manufacturers pursue better quality of radio performance with desired coverage, such as, for example, 2%. A low EVM can be achieved by having a large peak-to-average ratio (PAR) clipping threshold on a radio to minimize clipping, which is a significant source for EVM. In the present disclosure, the PAR clipping threshold is also referred as “power headroom.”
The 3GPP standards define the term “p-a” which refers to the power offset for Resource Elements (REs) at Physical Downlink Shared Channel (PDSCH) type A symbols toward cell reference signal (CRS) power. The type A symbols are the Orthogonal Frequency Division Multiplexing (OFDM) symbols without cell-specific reference signals. In the present disclosure, the terms “p-a value,” “p-a power offset value” and “power offset value” are used interchangeably. The 3GPP standards allow different p-a values to be configured for different wireless devices, therefore making it possible to have different power offsets for the downlink physical channel of different wireless devices. P-a values can be configured for any ranges, including, for example, {−6, −4.77, −3, −1.77, 0, 1, 2, 3}. An offset may also be applied to the type B symbols with p-b configurations. The type B symbols are the OFDM symbols with cell-specific reference signals. The parameter p-b is essentially the energy per RE ratio between that for type B symbol REs and type A symbol REs.
As described above, larger power headroom can result in lower EVM. However, increasing power headroom may result in reducing the radio transmission power for a given radio. One way to reduce transmission power is to maintain the power share and reduce power for all users. However, in this case, the cell coverage is adversely impacted. If a cell is designed for medium or large coverage, or its radio is also used with other technologies that need a share of the total available power, reducing the needed maximum power will reduce the cell coverage from the initial base station deployment plan, which will increase cell edge wireless device drop rates. More advanced radios could be manufactured to provide high power with low EVM but this will result in a higher cost for customers who would have to upgrade their radio hardware. Customers may not be willing to replace their existing radios with newer, advanced versions, just so the radios can support HOM in an already existing cell. If radios cannot provide enough radio power headroom as required for low EVM, downlink data transmission using 256 QAM may result in a high BLER due to radio power clipping. Thus, such modulation is not recommended. Further, HOM-capable wireless devices would lose their advantage for providing higher throughput and base stations would not obtain the benefit of potentially higher downlink data transmission capacity.